Nuclear power plants are currently required to store their spent fuel assemblies on site. Storage is accomplished by placing the spent fuel assemblies in closely packed fuel storage racks located at the bottom of on site storage pools. To provide maximum storage space, the storage racks contain a large number of adjacent storage cells, each cell being capable of accepting a spent fuel assembly. The walls of the cells include a neutron absorber to avoid critically and to permit the close packing of the nuclear fuel assemblies. This type of storage has been in use for over 15 years and in many sites the neutron absorber in the cell walls has begun to deteriorate. In order to extend the period over which the fuel assemblies may be stored in this manner, it is necessary to either replace the neutron absorber in the cell walls or to add an additional neutron absorber to the cell or the fuel assembly.
Although there are a wide variety of neutron absorbers, as well as methods for their fabrication and installation which theoretically could be applied in this application, there are generally no prior art neutron absorbers or methods capable of permitting on site installation in an economically feasible manner. The economic aspect of the installation of a neutron absorber is one of the most important because the retrofitting of the nuclear fuel storage racks at one site alone can cost tens of millions of dollars. The high cost is due in large measure to the great expense for new storage rack design, fabrication, licensing, and installation, as well as the expense for old rack removal and radioactive waste disposal incurred during this removal.
An example of an economically unsuitable prior art approach which could technically be applied to address the retrofitting process described above is contained in U.S. Pat. No. 4,787,029. In this patent, a fuel rack is described that is designed to store closely packed fuel assemblies. Within this fuel rack, a neutron absorber is encased in the cell walls that surround each spent fuel assembly To apply this fuel rack to a retrofitting application, all the spent fuel assemblies in an old installed rack would first have to be removed and placed in temporary storage. Following this, the old rack would have to be removed and disposed of as radioactive waste. Finally, the new rack would have to be installed and the spent fuel assemblies would then have to be placed in the new storage rack, all at great expense.
A second example of an unsuitable prior art approach for this retrofitting application is provided by U.S. Pat. No. 5,198,183. This patent illustrates a neutron absorber that may be inserted within the fuel assembly itself. The application of the neutron absorber described in this patent is also not economically feasible for retrofitting applications because it would require the steps of retrieval of the spent fuel assemblies from the underwater storage rack, the modification of the fuel assemblies by the installation and locking in place of the neutron absorber within the fuel assembly, and finally the return of the modified fuel assemblies to their cells in the underwater fuel rack, all of which would have to be carried out on site by skilled operators using specialized remotely operated equipment.
A third example of an unsuitable prior art approach for this retrofitting application is provided by a commercially available thick neutron absorber formed of boron carbide in an aluminum matrix. This neutron absorber is typically 0.1 to 0.2 inch thick and is recommended for installation about all four sides of a fuel assembly. This recommendation may be prompted by the relatively poor absorption properties of boron. By specifying the placement of the thick neutron absorber about all four sides of an unmodified fuel assembly, it becomes difficult, if not impossible, to install this absorber while the fuel assembly remains in a storage rack because there is usually insufficient clearance between the fuel assembly and the wall of the storage rack cell to accept such a thick walled neutron absorber. In an attempt to compensate for this problem, it is recommended that the fuel assembly be modified by retrieving it from the storage rack and removing the flow channel to make room for the thick walled absorber. If such a procedure is followed, the user must incur the cost of retrieval of each spent fuel assembly, disposal of the radioactive flow channel, installation of the new neutron absorber and return of the fuel assembly to the fuel rack. In attempting to solve the fuel assembly storage problem, this approach has created another nuclear waste storage and disposal problem which generally cannot be carried out on site.
In addition, the thick walled neutron absorber presents a potential failure mode over its lifetime. This absorber is first fabricated as a single plate which is then folded to have a rectangular cross section. The area in which the fold takes place contains the neutron absorbing material. Microscopic cracks occurring along the line of the fold could later develop into leaks which could reduce the effectiveness of this type of neutron absorber. Unfortunately, this is the exact problem that this absorber was designed to correct.
These and other limitations of the prior art are overcome by the present invention which is described in the following detailed specifications.